Guidelines for Updating and Deleting Data
The UPDATE and DELETE statements used in the previous sections all have WHERE clauses, and there is a very good reason for this. If you omit the WHERE clause, the UPDATE or DELETE is applied to every row in the table. In other words, if you execute an UPDATE without a WHERE clause, every row in the table is updated with the new values. Similarly if you execute DELETE without a WHERE clause, all the contents of the table are deleted.
Here are some best practices that many SQL programmers follow:
Use with Caution The bottom line is that MySQL has no Undo button. Be very careful using UPDATE and DELETE, or you'll find yourself updating and deleting the wrong data.